
DIGITAL WAVEGUIDE 2
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Back-fed fractional delay line with the following advanced features making it particularly suitable for delay-based / Karplus-Strong synthesis:

 
-Fractional delay support by all-pass interpolation ensuring smooth pitch resolution and flat spectrum
 response for every pitch

-Pitch can be modulated or switched abruptly without clicks or other artifacts

-Supports negative feedback

-Feedback is adjusted on a linearity-corrected scale

-Feedback auto-scaling, for all notes to have the same decay time

-HF Damp filter (LPF) in the feedback line for realistic simulation of plucked strings

-Automatic pitch adjustment in function of the Damp factor

-Builtin automatic input DC filter with variable cutoff for DC peak abatement

-Removal of ground spectrum for a cleaner sound

-Every parameter is properly scaled in function of sample rate, for the module to always sound indentically

-Suitable for polyphonic usage

-Infinitely more CPU efficient than the old Digital Waveguide, which has been completely re-engineered and re-written from scratch



Back-fed delay can be expressed by the following equation:

 y[t] = x[t-d] + fb * y[t-d]

where x is the input signal, y is the output signal, d is the delay amount in samples and fb is the feedback factor. The result is *similar* to that of a feedback comb filter, with the difference that harmonic peaks with both positive and negative feedback factors are always *added* to the original signal (i.e resonated). This symmetrical behaviour makes back-fed delay decisely more suitable for Karplus-Strong / delay-based sound synthesis than a comb filter.


NOTE ABOUT DC FILTERING

When using a positive feedback, the delay line will naturally produce equally spaced spectral peaks at positions NF, where F is the resulting pitch frequency and N is an integer starting from zero: this implies that a DC peak will always be present. A suitable highpass input filter is included, whose cutoff frequency is always half of pitch frequency.


PIN DESCRIPTION

.Power Switch
When Input goes STATIC, the module will become idle once the delay line stops ringing. This can happen or not, however, depending on feedback settings and other hard to predict conditions. In order to save CPU power, the module can be completely turned off when not in use by toggling this switch

.Clr
Inits the delay buffer and all filters variables to a fresh state. Any decaying sound is interrupted.

.Input
Input audio signal. Static updates are not supported, no to saturate the waveguide with DC (every static input value before going idle is zeroed)

.Pitch
Pitch in octaves

.FB
Feedback amount: -10 = -100%, 10 = 100%.
Please note that this value does NOT correspond linearly to the actual feedback factor from the formula above: a linearity correction is applied for easier adjustment.
Positive feedback results in a comb filter with all harmonics (harmonic at zero is filtered out if DC Filter is enabled), while a negative feedback results in a comb filter with odd harmonics only and half the specified pitch (no DC peak is present and the DC filter is not active when feedback is negative)

.Auto Scale FB
Automatically scales feedback factor in function of pitch, for all notes to decay with the same speed. Keep in mind that when a non-zero Damp factor is specified, higher pitched notes will tend to decay faster no matter what

.Remove GND
Removes residual input signal (ground signal) from the output spectrum, for peaks truly dipping at zero and cleaner sound

.Damp
A-dimensional feedback HF Damp value (automatically adjusted in function of sample rate). 0 = no damp, 10 = 100% damp

.DC Filter
Enables automatic abatement of the DC peak when a positive feedback is specified

                        
